What IS the Spacetime fabric?

I've been reading a lot about gravity lately, trying to understand it as best I can. When I think of gravity, I find it easiest to imagine the rubber sheet analogy to explain the spacetime curvature. Gravity is supposed to be caused by the curvature of spacetime. So the thing that I have no insight on is the 'fabric' of spacetime. What is it? It's talked about like it's a physical object in many texts that I've read. "A tear in the fabric of spacetime." You need something to tear or put a hole in, right? A spinning black hole will spin spacetime near the event horizon - spin what? It's also the medium that gravitational waves travel through. This again requires (in my mind) some kind of physical object, ie. sound waves travel through air - air is the physical object.

Another puzzling thing I've seen several times is the idea that the spacetime fabric travels faster than c as the universe expands and has done so since the big bang. The fact that it has always been faster than c satisfies the theory that nothing can CROSS the light barrier, but if the spacetime fabric is some sort of physical object, wouldn't it have mass? How can something with mass go faster than c - that would require infinite energy, wouldn't it? Would a (hypothetical) observer who is outside of our spacetime see the universe and everything in it moving faster than c (relative to their stationary position outside of spacetime) with the 'flow' of the spacetime fabric?

I'd appreciate any insight any of you could provide me on this. Any links would be very helpful as well.

Thank you Wolram - this article did provide some insight as well as a place to start. This article suggests that spacetime is made of one of three things:
1. Graviton particles (has the graviton ever been detected?)
2. Cosmic strings
3. Quantum foam
I've done reading on 2 & 3 and am fascinated by both, but it never occured to me that either of these could actually BE the spacetime fabric. Still, I'd appreciate any additional insight on this subject that any of you can provide. I'm still curious about my second question in the original post.

Curved spacetime often gets called the aether of Einstein. But it is a little misleading, since the aether of old is out the window. To once again quote the Ask an Astronomer website:

"....First of all, space-time is not a fabric. Space and time are not tangible 'things' in the same way that water and air are. It is incorrect to think of them as a 'medium' at all. No physicist or astronomer versed in these issues considers space-time to be a truly physical medium, however, that is the way in which our minds prefer to conceptualize this concept, and has done so since the 19th century. Back then physicists talked of an ether. Today we know that ethers of the kind that behave like a physical medium are simply not present.

We really do not know what space-time is, other than two clues afforded by quantum mechanics and general relativity. General relativity as developed by Albert Einstein, says, and this is a direct quote from Einstein, that

"Space-time does not claim existence in its own right, but only as a structural quality of the [gravitational] field"."

The gravitational field is spacetime. It can exist in the absence of matter and energy as a field. Lee Smolin briefly covers the concept of the field in "Three Roads to Quantum Gravity".

the general theory of relativity says that nothing can travel faster than light in spacetime, but not forbids spacetime itself to surpass this limit
i agree that spacetime has to be composed of some substance, after all gravity waves are ripples of spacetime

"Space-time does not claim existence in its own right, but only as a structural quality of the [gravitational] field"."[/i]

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this is a good quote
can you supply a source
I've seen Einstein quoted to this effect in other places but
have never managed to track down the book or article or interview by him where he said it
always someone else quoting einst. without a precise reference

Eh, thanks for the Einst. quote.
I found a reference to a piece he wrote in 1952
and appears in the umpteenth edition of one of his books
so I posted the link (to a longer passage with that quote in it)
in math forum

Yeah it was something he added in 1952 to the 15th edition of that book.

Eh, the quote you raised opens up the Great Wormcan of Relativity
Rovelli spends a couple of chapters discussing it including
how Einstein got hung up on it for three years 1912-1915
(called the problem "the meaning of the coordinates")

Rovelli's draft book online at his website has the best philosoph
discussion of these questions I know, you may know others

crux is that in general relativity the field has physical meaning
but the equation relating it and matter is invariant under smooth deformations so that

space-time points have no physical meaning---do not exist

there is no Newtonian absolute space and time upon which
these fields are defined

it is not like in 9th grade where you got handed a piece of absolute graph paper and drew curves and stuff on it

the theory is background-independent

so any background you use is just a provisional convenience and replaceable at whim by a smooth deformation

points in spacetime have no meaning, only EVENTS like
the intersection of two worldlines have meaning
because a crossing of paths remains a crossing of paths even after a smooth deformation ('diffeomorphism')

the main equation of GR is "diffeomorphism-invariant" and thus the theory is background independent and cannot be based on
some arbitrarily chosen absolute space-time

this has been hard for everyone to assimilate

and has interfered with attempts to build a quantum GR
because quantum theories tend to be built on an absolute space-time (the 9-th grader's piece of graph paper, or the world as recommended to us by Messers Newton and Minkowski)

If the history of science issues interest you, do you have a link to Rovelli's book----the history of western conception of space and time is fascinating and the whole thing sort of comes to the fore
in quantizing GR.

I define substance like something that exists, in contraposition of a hypothetical pure vacuum. In a pure vacuum gravity waves cannot exist (there's nothing to wave). For example in LQG, the substance can be the same loops. In string field theory that substance can be the string fields (though I'm not sure in this case)

Originally posted by meteor In a pure vacuum gravity waves cannot exist (there's nothing to wave).

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One of the hardest things to accept in modern physics is that waves can propagate without a medium.

The idea first comes in with Maxwell's Equations. An EM wave is self propagating , even in a vacuum. Put another way, the speed of an EM wave is not related to the pressure of anything, just two constants, the electric permittivity and magnetic permeability of free space.

What may help in understanding spacetime is that it is 4D whereas our sense are only used to 3. When we refer to spacetime 'bending' it bends in 3D, it may be flat in 4D.

To appreciate this I like the analogy Kip Thorne uses in 'Black Holes and Worm Holes". We assume that light has constant velocity in vacuo, more precisely it is invariant between inertial frames, and follows the shortest distance between two points. We also know that gravity causes things to accelerate in it's presence. So what happens when light passes through a gravitational field?

The answer to the conundrum is that it changes direction, the speed is scalar so does not change, but change direction you change velocity and it accelerates. But that means light follows a curved path in the prescence of gravity.

What we are seeing is light following the shortest path in 4D, which to us is a curve in 3D. The path it follows is spacetime. It is purely a geometric construct.

I think that the comparation of light with gravity waves is erroneous
Light is not a vibration of some pre-existing field. It's a self-propagating wave with electric and magnetic fields continously inducing one another
Gravity waves are vibrations of some pre-existing field: the gravitational field (aka spacetime)

quote:
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Originally posted by thed
What may help in understanding spacetime is that it is 4D whereas our sense are only used to 3. When we refer to spacetime 'bending' it bends in 3D, it may be flat in 4D.

Originally posted by ShadowKnight My senses are certainly only used to 3D
Can anyone out there explain this analogy to me using a 2D / 3D example? I feel like I'm almost to an understanding but the light hasn't fully come on yet.

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For starters, take the case where we just swap around what thed says (he may have unintentionally switched 3D and 4D):

[[When we refer to spacetime 'bending' it bends in 4D, it may be flat in 3D. ]]

This is the picture you get in the typical review article by a cosmologist these days (Michael Turner, Ned Wright, Charles Lineweaver, Plionis, Eric Linder, whoever).
If a reputable cosmologist writes a review of the present model he will say that the consensus is large-scale "spatial flatness" that is flat in 3D which probably means infinite in extent but all we can say from observations is 3D flatness

and he will say "expanding" which means 4D curvature

So then to respond to Shadowknight, how to picture this
consensus view (spatially flat expanding, therefore curved in 4D)

For starters look at a 2D space-time diagram with 1D space and one dimension of time and you see all the worldlines of individual galaxies are curving outwards as the distances between them increase.
the galaxies are sitting still in the space around them (in these simple pictures) but getting farther apart and so the geodesics they follow (straight lines in terms of spacetime geometry, or straightest possible anyway) are curved---when we make a flat picture of it.

You can find plenty of these 2D spacetime diagrams in Ned Wright's cosmology tutorial at the ucla site, or Lineweaver's tutorial at the caltech site.
Figure 1 on page 6 of Lineweaver is, I think, especially good
I would suggest downloading the whole paper

Originally posted by meteor
I define substance like something that exists, in contraposition of a hypothetical pure vacuum. In a pure vacuum gravity waves cannot exist (there's nothing to wave). For example in LQG, the substance can be the same loops. In string field theory that substance can be the string fields (though I'm not sure in this case)

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The problem with classic concept of a vacuum was not the lack of substance, but the fact it was seperated and had independent existence from the other properties of the field. Even so, such a volume of pure vacuum would still be reducible to lines. Lines are not made anything, as they are just space, but from the same standpoint, string and loops are not made from anything either. The difference is, while length would be the only property we would ascribe to lines in classic Euclidean space, lines or strings have other fundemental properties. They cannot be sepereated, hence you cannot have "space" with any independent existence of the field, which is the composite of all the properties.

This is just philosophy. But it helps if you take a concept of the classic vacuum, and list the properties it has. Then compare it to the properties of something deemed to be a "substance".